Granular media filtration is a process employed to remove material suspended in water or other fluid. Such suspended material is not readily separable from the fluid by gravity forces owing to insignificant density differences between the suspending fluid and the suspended solids. The problem is overcome by forming the suspended solid particles into clusters or aggregates. Flocculation is the process most often utilized to form aggregates of solid particles suspended in a fluid. These aggregates are formed as a consequence of velocity gradients which bring the particles in contact with one another.
In state of the art filtration, flocculation is normally an operation which occurs immediately ahead of filtration, although often with a gravity sedimentation step therebetween. In other words, suspended particles in a liquid are first aggregated to form settleable masses, most of which are separated in the gravity sedimentation step. The residual suspended particles, which have density differences relative to the suspending liquid insufficient to effect gravity separation in a reasonable period of time, remain in the suspending liquid and are separated by the subsequent filtration step.
Flocculation and gravity sedimentation, then, remove the bulk of the suspended solids, while subsequent filtration removes the residual suspended particles.
Granular media filtration is a complex process in which the removal of the suspended material can be attributed to a number of different forces: sedimentation; impact; physical Van der Waal forces; chemical bonding; absorption; and screening. The effective size of the granular media is selected to be as small as possible without resulting in rapid plugging of the filtration media. Two stage granular media filters are commonly used in the filtration process: the first stage consists of a relatively coarse coal material having an average particle size in the vicinity of one millimeter, while the second stage is composed of a granular sand material having an average particle size of approximately 0.1 millimeter. Such two stage filters generally exhibit longer filtration runs between backwashes in that large suspended particles which would rapidly plug the fine sand media are removed by the coarse media in the first stage, while the finer material is removed in the fine sand stage.
Filtration is a periodic process which of necessity must be coupled with a backwash cycle that effects removal of materials entrapped in the media during filtration. The most common method of backwashing the media requires an upflow of fluid which expands the filter media bed and induces hydrodynamic forces around each media element to effect both cleaning of the media and transport of the removed suspended material from the filter. Simple upflow and expansion of the bed is generally insufficient to effectively clean the filter media. Therefore, additional steps are taken to clean the media, such as mechanically raking the surface of the expanded filter bed, bubbling air through the filter bed and spraying the surface of the filter bed with high-velocity wash water.
The backwashing cycle imposes limitations upon the practical design of the filter bed. It is generally recognized that a coarse filtration media results in a longer filter run due to much better distribution of filtered solids throughout the filter bed before backwash is required. However, the use of such coarse media has not been practiced as the backwash water requirements are excessive.
Such a system requires a backwash water volume approaching the filtered water volume, rendering the system impractical. As a consequence, the above mentioned characteristic two-stage media sizes have been employed in commercial filters to ensure that the backwash volume required is limited to less than 5% of the filtered water volume.
The limitations imposed on the effective filter media size also delimit the concentrations of suspended solids that the filter can absorb from the waste-water, due to the rapid plugging of such beds. Thus granular media filtration has been practically limited to flows having less than 100 milligrams of suspended solids per liter of wastewater. It has been demonstrated that at suspended solid concentrations above such a value, the backwash water volume again approaches the filtered water volume.
In contrast to state of the art filtration systems, the present invention combines the distinct advantages of a coarse filter media with an efficient backwash system. To achieve the maximum benefits of the invention, it is desirable that the fluid to be treated is coagulated but not flocculated prior to application to the filter medium. The coagulated fluid is impacted upon the coarse filter medium where flocculation occurs. When an individual media particle is no longer able to absorb flocculated solids, subsequent flocculated suspended solids in the fluid effectively bypass this media particle and are forced through the media to subsequent elements. Through such a scouring process, plugging of the filter media will not occur and the entire filter bed can be used for storage of the suspended solids. When the filter bed becomes completely saturated with solids, the solids will "breakthrough" the filter bed and the filter bed with require backwashing.
To achieve the above objective during the filtration cycle, it is necessary to use a coarse granular media, such as sand having a size in excess of 1 millimeter and preferably larger than 3 millimeters. Such media requires a very high upflow velocity of clean fluid to achieve the filter bed expansion necessary in the backwashing step. Unfortunately this makes the coarse filter media impractical as almost the same volume of cleansing water is required to clean the filter as was filtered beforehand. The backwash cycle as practiced in accordance with this invention circumvents this problem by utilizing recycled backwash water to satisfy upflow requirements to expand the coarse media filter bed and separate the accumulated solid material from the filter media.
This initial high upflow rate is used in conjunction with a filter media cleaning step in which an ejector system draws large solids and media particles into a high velocity stream of cleansing water. This special cleansing cycle scours media particles and breaks up "mudballs" of solid materials. The ejector system is an integral part of the primary upflow cleansing system, which uses clarified water as a cleansing water. Water and suspended solids are drawn off above the upper level of the filter media during the backwash, and fed into a clarifier. The clarifier effectively separates suspended solids from the water, and recycles clarified water back into the upflow cleansing system. Thus, the present invention uses a two stage cleansing process which effectively cleans the coarse filter media while recycling the cleansing water to make the entire backwash cycle practical in terms of water use. As a final polishing step, filtered water from a clear water tank is injected into the bottom of the filter media to remove any residual materials from the bed. In such a manner, the net backwash water volume is held to less than 2% of the filtered water volume.
The flocculation/filtration system as described above must be considered integrally with the backwash cycle in that any granular media filtration system involves a filter media cleaning step in conjunction with a filtration step. The significant improvements of the present invention compared to state of the art filtration systems are primarily attributable to these two integral cycles: the use of a uniform coarse filter media allowing practical intramedia flocculation and solid storage heretofore not attainable in state of art filters, and a method of filter backwash cleaning which is especially applicable to the use of such a coarse media. The cleaning cycle disclosed accomplishes effective cleaning of coarse filter media while using no more and in many cases less backwash water than that required by conventional state of the art filters.